1. Field of the Invention
The present invention relates to a novel lubricating oil composition, and more specifically to a lubricating oil composition having high heat resistance, high oxidation stability and excellent lubricating properties, useful as a lubricating oil for internal-combustion engines, automatic transmission gearboxes, dampers, power steering units and the like, particularly useful as a lubricating oil for internal combustion engines.
2. Description of the Related Art
Lubricating oils have been used for internal combustion engines, and for driving units and gears such as automatic transmission gearboxes, dampers and power steering unit in order to smoothly operate them. In particular, lubricating oils for internal-combustion engines (engine oils) not only lubricate various sliding portions such as a piston ring, a cylinder liner, bearings for a crank shaft and a connecting rod, and a valve-operating mechanism including a cam and a valve lifter, but also cool the inside of the engines, clean and disperse those products which are produced by combustion, and prevent the rusting and corrosion of the engines.
Thus, the lubricating oils for internal-combustion engines have been required to have a great variety of properties. Moreover, due to the recent trend toward high-performance, high-output internal-combustion engines and more severe operating conditions, the lubricating oils are required to have higher quality. In order to meet this requirement, various additives such as an antiwear agent, a metallic detergent, a nonash dispersant and an antioxidant are incorporated into the lubricating oils for internal-combustion engines.
It is particularly important as the essential function of the lubricating oils for internal-combustion engines that the lubricating oils can ensure the smooth operation of the engines under every condition to prevent the wear and seizure of the engines. The parts of the engines to be lubricated are, in most cases, under the fluid lubrication condition. However, the valve train, and the top and bottom dead centers of a piston tend to be under the boundary lubrication condition. Antiwear properties under the boundary lubrication condition are generally imparted by the addition of zinc dithiophosphate (ZnDTP) or zinc dithiocarbamate (ZnDTC).
Energy loss at the friction parts of internal-combustion engines which are lubricated by lubricating oils is great. For this reason, a lubricating oil to which various additives including a friction modifier (FM) are added has been used in order to reduce the friction loss and to decrease the fuel cost (e.g., Japanese Laid-Open Patent Publication No. 23595/1991). Lubricating oils for automotive internal-combustion engines are used at various temperatures, at various revolutions per minute and under various loads. Therefore, in order to further improve the rate of fuel consumption, it is necessary that the lubricating oils be excellent in friction properties under a wide range of conditions under which they are used.
Besides the above-described properties, high heat resistance, high oxidation stability and moderate viscosity characteristics can be mentioned as the properties required for the lubricating oils for internal-combustion engines.
The present invention is directed to meeting these requirements. An object of the present invention is therefore to provide a lubricating oil composition having excellent lubricating properties, high heat resistance, high oxidation stability and moderate viscosity characteristics, particularly useful as a lubricating oil for internal-combustion engines.